(54) PROCESS FOR THE PREPARATION OF PRINTING FORMS(71) We, HOECHST AKTIENGESELLSCHAFT, a Body Corporate organized according to the laws of the Federal Republic of Germany, of 6230 Frankfurt/Main 80, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a process for the preparation of printing forms.
The invention provides such a process by electrostatic charging of a photoconductor layer comprising at least one organic photoconductor, at least one sensitizing dyestuff, and if required or desired at least one binder, coated on and adherent to a conductive base, followed by image-wise exposure, application of a toner to the latent electrostatic image, fixation, decoating, and possibly etching of the image-free areas.
Significant advances in the art of printing plate technology have occurred in recent years. Printing plates such as have been employed in lithographic offset or direct printing processes are normally prepared by the image-wise exposure of a photosensitive coating which has been applied to a suitable support. Typical of such coatings are the so-called positive acting quinone diazides, as for example disclosed in German PatentSpecification No. 854 890, which undergo photodecomposition in areas of the coating exposed to a source of actinic light, which exposed areas may subsequently be removed by treatment with a liquid developer solution in which only the photodecomposed areas are soluble.
The negative acting coatings, on the other hand, undergo a photohardening or photopolymerization in those areas exposed to actinic light and it is the unexposed areas of the coating that are subsequently removed by appropriate developer. Representative of such negative acting materials are para quinone diazides such as disclosed in GermanPatent Specification No. 960 335, or condensation products of diazonium salts such as disclosed in U.S. Patents No. 3 679 419, No. 3 867 147, and No. 3 849 392.
Offset plates have also been prepared by electrophotographic methods. Such plates are normally composed of a finely divided inorganic photoconductive material such as zinc oxide or cadmium sulfide dispersed in an ink-repelling binder material coated on a suitable base material such as paper or a film. These plates are imaged by the normal electrophotographic process involving forming a uniform electrostatic charge on the surface of the plate, exposing the charged plate on an electrically conductive support to an image pattern of electromagnetic radiation, developing the resulting electrostatic image pattern by contact with an electroscopic liquid or solid oleophilic developer, and fixing the developed image by drying or heating. The resultant imaged plate may be then used as a master for offset lithographic printing.
Other processes for the electrophotographic production of printing plates are described in British Patent Specification Nos. 944,126, 951,106, and 952,906 where, inter alia, oxazoles and oxadiazoles are employed as photoconductors in compositions that are alkali-soluble, solubility in alkaline media being imparted, in certain instances, by an alkali-soluble binder.
Other techniques for preparing printing plates involve the preparation of an intermediate photographic silver halide film master which is then used to expose an offset printing plate, or the preparation of relief-type plates using radiation of high energy to engrave or etch the plate and form minute holes or depressions in image configuration.
Because of the increased use in recent years of electronic methods for recording, storing and generating information by, for example, computers, cathode ray tubes and facsimile devices, there have been some proposals for modification of printing plate manufacturing techniques and the compatibilization of plate making processes with the newer technology for generating image information. For example, U.S. Patent No.
3 549 733 discloses the use of a modulated high intensity 30 watt carbon dioxide gas laser to image a printing plate wherein polymeric material on the plate surface is decomposed to form ridgeless depressions, thus forming a relief plate. U.S. Patent No.
3 506 779 discloses a laser beam typesetting apparatus for forming relief plates wherein a high intensity 100 watt carbon dioxide laser is utilized to remove plate material from the plate surface by vaporization. U.S. Patent No. 3 664 737 discloses a printing plate recording system involving direct laser exposure of diazo sensitized printing plates which are subsequently developed by conventional development methods.
Systems such as described in U.S.. Patents No. 3 549 733 and No. 3 506 779 require relatively high powered output lasers in order to accomplish the work of etching or deforming the recording surfaces. Aside from the high energy requirements of such lasers, there are attendant problems in providing adequate cooling means which adds bulk and expense to the apparatus in which such lasers are embodied. Even with the system disclosed in U.S. Patent No. 3 664 737, a relatively high powered laser would be required to suitably expose printing plates where the photosensitive materials used were quinone diazides diazo compounds, diazonium salts or aromatic azido compounds.
These materials generally require a minimum plate energy in the order of at least 20 millijoules/cm2 of plate surface for satisfactory photochemical exposure. This would be a limiting factor in terms of the output plane scan speed in an apparatus such as disclosed in U.S. Patent No. 3 664 737 unless a relatively high powered laser device were employed. On the other hand, if the photosensitive medium used in this patent were an electrophotographic material like zinc oxide coated paper or silver halide film, considerably less energy would be required for satisfactory exposure, but the advantages of imaging directly onto a durable printing plate which can be readily developed by image fixation and removal of the light-exposed photosensitive coating would be lost.
Further, an exposure system has been proposed in which laser beams are used for exposing electrophotographic material (U.S. Patent 3 909 254). In this process, the photoconductive surface is image-wise exposed prior to electrostatic charging in order to retain the conductivity of the areas struck by light. According to what is disclosed in the patent, image-wise differentiation is produced by charging, and the latent image may then be converted into a visible, fixed image by the application of a toner and further processing in known manner.
For this purpose, however, it is necessary that a very high energy acts upon the surface of the plate in order to create lasting conductivity, and this energy is emitted by a laser at an intensity of 500 watt/cm2.
The present invention provides a process for the preparation of a printing form comprising exposing an electrostatically charged electrophotographic plate comprising a conductive base material carrying a layer of a photoconductive insulating composition comprising a sensititizing dyestuff, an organic photoconductor, advantageously at least one substituted oxazole, substituted triphenylamine, carbocyclic aromatic compound preferably having at least three fused rings, benzo-condensed heterocyclic compound, pyrazoline, imidazole, triazole, oxadiazole, homopolymer or copolymer of vinylanthracene, acenaphthylene or N-vinyl-carbazole, polycondensate of an aromatic amine' and an aldehyde, polycondensate of a substituted pyrene or penylene with formaldehyde, and if desired or required, a binder, exposing the charged layer to a modulated light pattern of a wavelength in the range of from 350 to 750 nm emitted from a laser of a power less than 1 watt, to form a latent image, applying a developer containing a toner to the image, fixing the resulting toner image, removing the layer in the image-free areas without removing the toner image, and, if desired or required, etching the image-free areas. The composition, in effect the photoconductor sensitized by the dyestuff, is one which is sensitive to light of the wavelength, which wavelength is in the range of from 350 to 750 nm, emitted by the laser.
The electrophotographic plate comprises a layer of thin uniform photoconductive insulating composition coated on and adherent to a conductive base material. The composition may comprise a mixture of two or more photoconductive compounds from the above list. As a highly condensed aromatic compound there may be mentioned, for example, anthracene.
The process of the invention makes it possible to use a laser having a power of less than one watt with sufficient power to deliver a light energy between about 10 millijoules down to one microjoule per square centimeter of the layer in normal operation.
In a preferred embodiment of the invention, the photoconductive insulating layer may be exposed to a modulated pattern of laser light having a power of from 5 to 20 milliwatts, delivering light energy from 1 to 500 microjoules per square centimeter of the layer.
By the process of the invention, the electrophotographic material may be directly exposed and turned into a printing form without the preparation of intermediate originals on silver halide film or zinc oxide paper, and cost-saving lasers may be employed. Lasers of not more than 1 watt energy output are inexpensive, have a low energy consumption, do not need any significant maintenance, need not be cooled, and have a relatively long working life.
Any laser emitting within the above mentioned wave length range may be used for the process of the present invention, e.g., a helium/cadmium laser, argon ion laser, aYAG laser, or a helium/neon laser. Helium/neon lasers and argon ion lasers are preferred.
Any suitable material may be used as conductive base material in the present invention, e.g., aluminum, zinc, magnesium, or copper plates, also multi-metal plates, cellulose products, e.g., special papers, cellulose hydrate, cellulose acetate, or cellulose butyrate films, the latter preferably in a partially saponified form. For certain purposes, synthetic materials, e.g., polyamide films or metallized plastic films may also be used as support.
Aluminum foils which have been subjected to a surface treatment have proved to be particularly advantageous. The surface treatment consists of a mechanical or electromechanical roughening of the surface which may be followed, if desired, by anodization or a polyvinyl phosphonic acid treatment according to German Offenlegungsschrift No.
16 21 478. In this manner, longer runs and a reduced susceptibility to oxidation are achieved.
Photoconductors suitable for use in the photoconductive layers of the invention are known per se. Preferably, the substances known from German Patent No. 11 20 875 may be used, especially substituted vinyl oxazoles. As indicated above, other suitable photoconductors are, e.g., substituted triphenylamines, condensed aromatic compounds, e.g. anthracene, benzo-condensed heterocyclic compounds, and pyrazoline and imidazole degvatives. Further, the triazole and oxadiazole derivatives disclosed in German PatentsNo. 10 60 260 and No. 10 58 836 may be used; among these, 2,5-bis-(4'-diethylaminophenyl)-oxadiazole-1,3,4 is particularly suitable. Further, vinyl aromatic polymers, e.g., homopolymers of vinyl anthracene, acenaphthylene, N-vinyl-carbazole.The copolymers of these compounds may also be used, provided exposure causes differentiation in solubility, if necessary in combination with a resin binder.
Also suitable are polycondensates of aromatic amines and aldehydes as described in German Auslegeschrift No. 11 97 325, and resins obtained from polycondensates of optionally substituted pyrenes and/or perylenes with formaldehyde, as described inGerman Offenlegungsschrift No. 2137 288, preferably a polycondensate of 3-bromopyrene with formaldehyde.
In a preferred embodiment of the invention, the photoconductive compound is selected from substituted oxanoles, pyrazolines, imidazoles, triazoles, and oxadiazoles.
Among these, the photoconductive compound is preferably at least one oxazole compound of the general formula
in whichR represents hydrogen or an aryl, alkenyl, alkyl, substituted aryl, or heterocyclicradical,R1 represents hydrogen or an alkyl, alkyl amino-aryl or other substituted arylradicals, andR2 represents hydrogen or an aryl, or substituted aryl radical.
Preferred oxazoles are aminophenyl-substituted oxazoles of the general formula:
in which R is hydrogen or alkyl, R6 is aminophenyl or dialkylaminophenyl, R, is phenyl or substituted phenyl, Rs is hydrogen, alkyl, alkenyl or a heterocyclic radical, and Rg and Rlo are phenyl or substituted phenyl in one or both of which amino or dialkylamino is a substituent.
Alternatively, the photoconductive compound is preferably at least one substituted oxadiazole compound of the general formula
in whichR3 represents hydrogen or an alkyl, acyl, or cycloalkyl radical, andR4 represents hydrogen or an alkyl radical.
Provided their film-forming properties and their adhesive strength are appropriate, either a natural or a synthetic resin may be used as binder. Besides their film-forming properties, the electrical characteristics, the adhesion to the support, and the solubility characteristics affect the selection of the resins. Preferred binders are those soluble in, preferably acidic or alkaline, aqueous or alcoholic solvent systems. Relatively inflammable aromatic or aliphatic solvents are not preferred, for physiological and safety reasons. High-molecular weight substances carrying groups which render them alkalisoluble are especially suitable as binders. Groups imparting alkali-solubility are, e.g.
acid, anhydride, carboxylic, phenol, sulfonic acid, sulfonamide, and sulfonimide groups.
Preferably, binders with high acid numbers are employed because they dissolve with particular ease in alkaline-aqueous-alcoholic solvent systems. Copolymers containing anhydride groups may be used with particular success, the dark conductivity of the electrophotographic reproduction material containing them being low, despite good solubility in alkalis, because there are no free acid groups present.
Copolymers of styrene and maleic anhydride, such as those marketed by Monsanto under the designation "Lytron"'2', are particularly suitable. Phenol resins, e.g., those marketed by Hoechst Aktiengesellschaft under the designation "Alnovol"(R', are also useful.
The spectral sensitivity of organic photoconductors may be extended from the range of long wave UV light (about 350 to 450 nm) into the visible range by adding dyestuffs of various colors and belonging to various classes of compound as sensitizers.
The following dyestuffs, taken from the "Farbstofftabellen" by Schultz (7th Edition,Vol. 1, 1931) are mentioned as examples of effective sensitizers:Triaryl methane dyes, e.g., Brilliant Green (No. 760, page 314), Victoria Blue B(No. 822, page 347), Methyl Violet (No. 783, page 327), Basic Pure Blue, CrystalViolet (No. 785, page 329) and Acid Violet 6B (No. 831, page 351);Xanthene dyes, e.g., Rhodamines, e.g., Rhodamine B (No. 864, page 365), Rhodamine 6G (No. 866, page 366), Rhodamine G extra (No. 865, page 366), Sulforhodamine B (No. 863, page 364), and Fast Acid Eosin G (No. 870, page 368).
Phthaleins, e.g., Eosin S (No. 883, page 375), Eosin A (No. 881, page 374),Erythrosine (No. 886, page 376), Phloxin (No. 890, page 378), Bengal Rose (No.
889, page 378), and Fluorescein (No. 880, page 373);Thiazine dyes, e.g., Methylene Blue (No. 1038, page 449);Acridine dyes, e.g., Acridine Yellow (No. 901, page 383), Acridine Orange (No.
908, page 387), and Trypafiavine (No. 906, page 386);Quinoline dyes, e.g., Pinacyanole (No. 924, page 396), and Kryptocyanine (No.
927, page 397);Quinone dyes, and ketone dyes, e.g., Alizarin (No. 1141, page 499), Alizarin RedS (No. 1145, page 502), and Quinizarin (No. 1148, page 504). Further, cyanine dyes may be mentioned, e.g., Astrazone Orange R color Index No. 48040), AstrazoneOrange G (C.I. 48035), Astrazone Yellow 3G (C.I. 58055), Astrazone Yellow 5G (C.I. 48065) or Basic Yellow 52115 (C.I. 48060), and Malachite Green (C.I. 42000).
If, according to a preferred embodiment of the present invention, a helium/neon laser is used, Brilliant Green, Malachite Green, or Crystal Violet may be used, or any other dyestuff sensitizer which absorbs in a wave length range that includes the emission from the helium/neon laser at 630 nm.
The latent image may be made visible by means of a resin-containing developer, either in the dry state or dispersed in the form of a liquid developer.
In most cases, the developer substance is applied to the layer to be developed by means of a carrier. A mixture of iron filings used in the known magnetic brush or magnetic roll leads to favorable results.
Because of the properties of relatively clean working and high resolving power, liquid development has proved particularly advantageous in the present invention. The invention also provides a liquid developer containing a highly resistive liquid phase and a finely divided solid resin phase dispersed therein. Sometimes, however, it is of advantage if the solid phase includes a dyestuff. The particle size should be below 10 ,am. The solid substance may be ground in a ball mill.
Application of the developer may be performed in a manner known per se, e.g., by immersion or roller application.
After development of the latent electrostatic image with one of the afore-mentioned developers, the resin substance loosely adheres image-wise to the layer and may be fixed. Fixing is preferably performed by heating in an oven heated to the required temperature. Infra-red radiators of sufficient intensity are also suitable.
The duration of the heat treatment depends upon the photoconductive layer and the fixing properties of the resins present.
The image areas become insoluble in suitable solvents, i.e., in a preferred embodi ment of the invention they are insoluble in aqueous alkaline solutions.
The solutions may be applied to the layer by means, e.g., of a cotton pad. It is also possible to immerse the plates directly into the liquid. Also suitable for removing the non-image areas are appropriately constructed devices, e.g., those with applicator rolls for the liquids.
The differentiation into hydrophilic and oleophilic areas desired for offset printing, is thereby obtained, the toner image forming the olephilic areas and the bared support surface forming the hydrophilic areas.
After treatment with the alkaline solution, the printing plate advantageously is after-rinsed with water and, if desired, the hydrophilic properties are further enhanced by wiping over with dilute phosphoric acid solution. After inking up with greasy ink, prints can be made in planographic printing machines e.g., in a manner known per se.
It is also possible to produce printing plates for relief printing and, if desired, also for intaglio printing by subsequent etching of the bared support. Etching is performed in known one-step or multi-step etching machines.
The printing plates prepared in accordance with the invention may be capable of producing long printing runs.
The process according to the present invention may be used for the preparation of planographic printing forms suitable, e.g., for newspaper printing, and the device used for the laser exposure may be connected to a telecommunication system or a computer storage unit.
The invention is illustrated by the following examples:EXAMPLE 1.
A solution comprising 40 g of 2,5-bis-(4'-diethylaminophenyl)-1,3,4-oxadiazole, 47 g of a styrene/maleic anhydride copolymer, 10 g of chlorinated rubber, and 2.08 g of Astrazone Orange R, in 520 g of tetrahydrofuran, 330 g of methyl glycol, and 150 g of butyl acetate, is applied to a mechanically roughened aluminum foil of 100 Mm thickness. After evaporation of the solvent, the electrophotographic insulating layer is about 5 Mm thick. The sensitivity of the layer is within the blue spectral range, with a maximum at 480 nm. The energy required to discharge the layer from 450 V to a residual potential at 50 V corresponds to 60 pTlcm2 at 487 nm.After charging the plate to a surface potential of '-450 V, the plate is exposed to the radiation emitted by a 10 mW argon ion laser the beam of which is image-wise modulated to produce a negative image. The latent image is developed with a liquid toner obtained by carefully dispersing 1 g of a synthetic ester wax with a saponification number of 130-150 and a dropping point between 81 and 86"C in 20 ml of an isoparaffin in which 2 g of a soluble pentaerythritol resin ester (e.g. "Pentalyn"*, marketed by Hercules Powder,U.S.A.) are dissolved as a dispersing agent, and diluting the dispersion by adding 1000 ml of an isoparaffin with a boiling range from 185 to 210"C. 0.5 of soy lecithin are added to the developer to influence the charge.
The developed plate is then immersed for 1 minute in a solution of 35 g of sodium metasilicatehydrate in 140 ml of glycerol, 550 ml of ethylene glycol, and 140 ml of *-Trade Mark.
ethanol and then rinsing it with tap water while brushing it lightly. The resulting printing form yields high-quality prints with a resolution of 6 lines/mm (60 mesh screen) and runs of up to 100,000 copies.
EXAMPLE 2.
A solution comprising 40 g of 2-vinyl-4-(2'-chlorophenyl)-5-(4"-diethyl-aminophenyl)-oxazole, 47 g of a styrene/maleic anhydride copolymer, 10 g of chlorinated rubber, and 0.4 g of Brilliant Green in 510 g of tetrahydrofuran, 330 g of methyl glycol, and 150 g of butyl acetate, is applied to a 100 ,um thick aluminum foil which had been mechanically roughened by wire-brushing with a brush depth of approx.
3 am. After evaporation of the solvent there remains a 4 to 5 um thick photoconductor layer which absorbs within the red range of the spectrum, with a maximum at 630 nm, and becomes photoconductive. The energy required for discharging the layer to a residual potential of 50 V is 110 ,/lJ/cm2. The layer is charged in the manner usual for electrophotography with a corona device to a surface potential of s400 V and then image-wise exposed with a modulated 15 mW helium/neon laserhaving a wave length of 632 nm. The resulting electrostatic image of the original ismade visible by dusting it with a resin powder colored by the addition of carbon blackand is then fixed by heating it to 1500C and turned into an electrophotographic copywhich is fast to rubbing.In this manner, an electrophotographic copy is producedwhich corresponds to the original and the image areas of which are resistant to alkalinesolutions.
In order to convert the electrophotographic copy into a planographic printingform, the material is treated with a solution comprising 5 per cent by weight of monoethanolamine, 5 per cent by weight of diethanolamine, 10 per cent by weight of methylalcohol, 55 per cent by weight of ethylene glycol, 20 per cent by weight of glycerol, and5 per cent by weight of sodium silicate. After briefly rinsing with water and inkingwith greasy ink, copies may be printed from the resulting printing form.
WHAT WE CLAIM IS:1. A process for the preparation of a printing form comprising exposing an electrostatically charged electrophotographic plate comprising a conductive base material carrying a layer of a photoconductive insulating composition comprising a sensitizing dyestuff, an organic photoconductor as photoconductive compound, and, if desired or required, a binder, exposing the charged layer to a modulated light pattern of a wavelength in the range of from 350 to 750 nm, to which the composition is sensitive emitted from a laser of a power less than 1 watt and being capable of delivering to the layer a light energy between 10 millijoules and 1 microjoule per square centimetre of the layer, to form a latent image, applying a developer containing a toner to the image, fixing the resulting toner image, removing the layer in the image-free areas without removing the toner image, and, if desired or required, etching the image-free areas.
2. A process as claimed in claim 1, wherein said photoconductive insulating layer is exposed to a modulated pattern of laser light having a power from 5 to 20 milliwatts but sufficient power to deliver a light energy of between 1 and 500 microjoules per square centimetre of the layer.
3. A process as claimed in claim 1 or claim 2, wherein said photoconductive insulating layer is exposed by a helium/neon laser.
4. A process as claimed in claim 1 or claim 2, wherein said photoconductive insulating layer is exposed by an argon ion laser.
5. A process as claimed in claim 1 or claim 2, wherein said photoconductive insulating layer is exposed by a modulated helium/neon laser having a power of 15 milliwatts.
6. A process as claimed in any one of claims 1 to 5, wherein the photoconductive compound is a substituted oxazole, pyrazoline, imidazole, triazole, or oxadiazole.
7. A process as claimed in claims 1 to 5, wherein the photoconductive compound is at least one substituted oxazole of the general formula
in which
**WARNING** end of DESC field may overlap start of CLMS **.